1. Field of the Invention
This invention relates to a catalyst for hydrogenation of coal tar (hereinafter referred to as CT), coal tar pitch separated from CT (hereinafter referred to as CP), or heavier oil derived from coal sources or the like, the method of hydrogenation with use of such catalyst, and the method of producing high quality needle coke (hereinafter referred to as N-coke), or super needle coke (hereinafter referred to as SN-coke), from the hydrogenation product.
2. Description of the Prior Art
The art of hydrogenation of CT for the preparation of the high valued material from such hydrogenation product, such as coke required for the manufacture of graphite electrodes that may have a good performance under the quick melt conditions of the ultra high power (UHP) operation of the electrical furnaces, is already shown in the Japanese Laid-Open Patent Publication No. 122585/1984.
The art for improving the de-nitrogenation of the starting material derived from CT by the catalyst composed with various kinds of metal is described for example in the Japanese Laid Open Patent Publication.
However, there are no descriptions concerning the poisoning to the catalyst by the basic nitrogen compounds such as pyridine, quinoline or acrydine, which must necessarily be taken into consideration when using the starting materials derived from CT. Thus one may feel anxious about whether the stable catalytic action may be maintained during the operation over an extended time.
As the method of hydrogen treatment of the pitch-like material, it is also described about the catalyst which inhibit hydro-decomposition of the aromatic nuclei themselves in it to a lesser extent but selectively decompose the condensed aromatic hydrocarbons into a low molecular weight products (Japanese Laid-Open Patent Publication No. 198788/1982). Accordingly, this case may be specific in that there occurs no hydrogenation for CT as the medium of pitch like material.
The carbon material generally prepared by coking the starting coking material usually at the coking temperature range of 430.degree. to 470.degree. C. or thereabouts, known as raw needle coke (N-coke) or green needle coke (green N-coke) is composed of aggregates of graphite-like fine crystallites of hexagonal system with the mean size of the order of 1 nm. The properties of the N-coke required for the preparation of the high-quality graphite electrodes are dependent in a known manner on the orientation of and the binding force acting on these crystallites.
The formation of these crystallites are markedly affected in a known manner by the state of generation of the optically anisotropic mesophase spherules from which bulk mesophased are formed by coalescence of spherules and growth thereof finally resulting in the coke precursors upon heating the starting coking material.
On the other hand, the mesophase spherules are affected by such factors as the composition of the starting coking material, impurties that obstruct the growth of the mesophase spherules, and the coking conditions, so that it is by no means easy to specify the unique condition and structure for obtaining of the high quality N-coke.
Therefore, the essential conditions for the preparation of the high grade N-coke usable for the preparation of the graphite electrode usable for the purpose of UHP operation are the meticulous sorting or selection and the refining of the starting coking material.
For example, it is described in the Japanese Patent Publication No. 78201/1977 to separate or eliminate quinoline insolubles (QI) out of CP through the selection of the ratio of the aromatic solvents mixed with CP and being coked the resulting material by the conventional delayed coking to the N-coke. It is described in the Japanese Laid-Open Patent Publication No. 28501/1977 to eliminate the QI components out of the hydrocarbon material containing said QI components and the condensed cyclic hydrocarbon compounds by using a solvent the 95 volume percent (ASTM) of which has the boiling point lower than 330.degree. C. and the BMCI value of which is in the range of 5 to 70, then to remove the solvent and being coked the resulting product by conventional delayed coking to the desired N-coke.
It should be noted that the methods described in these two publications are intended for QI removal. When the starting coking material prepared by these two methods are used for coke manufacture it is possible to obtain the premium grade N-coke (PN-coke) in terms of CTE, however, swelling or puffing phenomena are undesirably observed in the preparation of graphite electrodes in accordance with the Lengthwise Graphitization System (LWG-system).
Such puffing phenomena is also seen to occur with the N-coke which is of substantially the same grade as that obtained from the starting coking material derived from petroleum sources. In the case of petroleum coke, however, such puffing is mainly ascribable to the sulphur contained in the coke and, in general, may easily be controlled by the addition of iron oxides as anti-puffing agent.
It is also known that the graphite electrode from the PN-coke manufactured from the material derived from coal sources is excellent in mechanical strength but slightly inferior in tenacity to the similar product derived from petroleum sources.
Although the reason for these defects is not known precisely, it is generally thought that both the desorbed gases from hetero atoms contained in the coke such as N, O or S and the texture of the carbon material are playing some part in the course of the graphitization of the electrodes.
Thus it may be surmised that the effect of certain ingredients may become predominant in QI free CT(QIF-CT) or QI free CP(QIF-CP) which favour the formation and growth of the good quality mesophase under a lower coking temperature while simultaneously allowing for development of the mesophase into the bulk mesophase with fibrous texture, but which obstruct generation of the bulk mesophase with fibrous texture when the coking temperature is increased, the coking speed of the ingredients in the QIF-CP or QIF-CT then becoming higher.
There is described in the Japanese Patent Publication No. 11442/1974 the method of modifying the CP by hydrogenation to a starting pitch material having a chemical structure likely to produce easily graphitizable needle coke. However, it is not possible to prepare the SN-coke even if the material produced in this manner is used as such as the starting coking material.
In the Japanese Patent Publication No. 41129/1976, there is described the method for the preparation of the pitch coke form the tar pitch derived from petroleum sources and that derived from coal sources. According to this method, the starting tar pitch is alkylated and thereafter modified in the presence of the hydrogenation catalyst.
However, by these methods, the QI components are still contained in the starting coking material, so that it is not possible to obtain the starting coking material for SN-coke schemed to provide by the present invention.
In general, the rate or reaction speed in the catalytic hydrogenation of various materials including QIF-CT or QIF-CP is determined to some extent by the diffusion of reactants into the catalyst pores.
From the above it will be appreciated that the larger pore size is effective to promote dispersion of the reactants into the active points in the pores. On the other hand, the larger is the pore size of the catalyst, the lesser becomes the active surface areas of the pore, so that the overall reaction speed is unavoidably lowered.
Also, with an oil with high asphaltenes contents and high aromaticity (fa) such as CT, the inside space of the pore is gradually occluded with the precipitated carbonaceous material, as the result of that the diffusion capacity into the pores is further lowered so that the desired effect at the initial stage of the catalyst is not expected to continue for any prolonged time.
As a means of increasing the active surface of the catalyst having a larger pore size, it may be feasible in principle to increase the pore volume. However, such increase in the pore volume results in declining the performance in strength of the catalyst thus causing breakage during catalyst charging into the reactor or powdering due to wear caused during operation so that industrial application may occasionally become an impossibility.